The present invention relates to speedometer-odometer apparatus in general and in particular so a speedometer-odometer apparatus comprising a calibrating means for use in vehicles in which highly accurate speed and distance measurements are required or desired.
In a motor vehicle, a conventional speedometer apparatus typically comprises a speedometer cable and a dashboard assembly including a visual indicating means. The speedometer cable interconnects the dashboard assembly to a transmission or drive line of the vehicle. In the dashboard assembly there is usually provided a magnet, a combination of gears and pinions and a needle or other speed indicating means. Typically the gears, which are driven by the speedometer cable, drive a series of rotatable wheels on which are imprinted numbers for accumulating mileage. The magnet, also connected to the speedometer cable, is rotatable and, when rotated, causes a movement of the needle in proportion to its rate of rotation for providing an indication of the speed of the vehicle. There are, of course, other arrangements possible.
Taking note of the fact that a visual readout of the distance that a vehicle travels and the speed at which it travels is a function of tire size, it will be appreciated that, since tire size changes continuously with tire wear, a speedometer-odometer calibration must be made regularly if a relatively accurate indication of speed and distance is to be maintained. The accuracy of the output of a speedometer and odometer apparatus also is affected by normal tire changes, as when winter and summer tires are interchanged on a vehicle, when tires become under and over-inflated or when the magnet in the speedometer becomes partially degaussed
To assist drivers to calibrate their speedometer-odometer apparatus, measured miles are posted at various locations along the highways. For convenience, a notice of an upcoming measured mile is generally provided some distance ahead of the beginning of the measured mile to warn the driver and allow him time to adopt and maintain a constant rate of speed. Presently, when abreast of the initial or starting post of the measured mile, the driver notes the odometer reading and the time. After passing the terminating post of the measured mile, the driver again notes the odometer reading and the time. If the speedometer-odometer apparatus is working properly, and the tire size, inflation, etc. correspond to the tire size, inflation, etc. for which the speedometer-odometer apparatus is calibrated, the driver will note as he passes the terminating post that precisely one mile has been added to the odometer reading and that precisely one minute has passed in time since the passage of the initial milepost if the speed adopted and maintained was 60 mph. If these are not the conditions observed by the driver, the driver must note the difference and make a determination as to the correction to be applied to calibrate the apparatus. It may be noted that an actual adjustment of conventional apparatus is generally not possible. Therefore, the calibration must be noted or remembered and applied mentally.
In motor vehicle regulation and speed enforcement, the accurate calibration of speedometers becomes very important. Presently, law enforcement officers employ an apparatus commonly known as the fifth wheel to calibrate their automobile speedometer apparatus. In use the fifth wheel is attached to the officer's motor vehicle and appropriate readings taken to calibrate the speedometer against the output of the fifth wheel. Needless to say, in addition to having to note the calibration on a chart and mentally applying it when involved in traffic control operatons, the use of a fifth wheel to obtain the calibration figures is time-consuming and expensive. Moreover, it may be noted, that when attempting to calibrate the speedometer in a vehicle using the measured mile described above or a fifth wheel, it is difficult to maintain a constant rate of speed over the mile.